you also have to be actively working the mine in order to discover a new resource.
Discovering new resources is a nice bonus, but I don't consider that bonus stronger than the health bonus forests bring.
As a general rule, if I can't build lumbermills, and I'll be working that hill anyways, I cut down the forest and build a mine. Otherwise, I leave the forest in place.
OTOH, the slower game speeds enhances the value of chopping a forest. Unit costs are proportionally smaller on slower game speeds, so you technically get more hammers by cutting it down, AND you're more likely to pop a resource with all those extra turns.
I've only played Marathon once, but I often play Epic games. In one game, my unit production city popped copper, iron, gems, gold, and coal during the course of a game... but they originally had jungles, so it wasn't like I had much of a choice.